<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
    <style>
        *{
            padding: 0;
            margin: 0;
        }
        .w{
            width: 500px;
            height: 500px;
            border: 1px solid red;
            /* display: inline-block; */
            /* vertical-align: text-top; */
            /* vertical-align: baseline; */
            /* vertical-align: text-bottom; */
        }
        .a{
            width: 50px;
            height: 50px;
            border: 1px solid red;
            margin: auto;
            display: inline-block;
            /* vertical-align: top; */
            /* vertical-align: bottom; */
            /* vertical-align: middle; */
        }
    </style>
</head>
<body>
    <div class="w">
        56556
        <div class="a">

        </div>
    </div>
</body>
</html>